1. Field of the Invention
The present invention relates to systems for slurry-based abrasive finishing and polishing of substrates, more particularly to such systems employing magnetorheological fluids (MRF), and most particularly to an improved system wherein the magnetic field is optimally shaped through novel shaping of the pole pieces and wherein the viscosity and flow rate of recirculated MRF is dynamically controlled through use of a novel capillary viscometer and inline flowmeter.
2. Discussion of the Related Art
Use of magnetically-stiffened magnetorheological fluids for abrasive finishing and polishing of substrates is well known. Such fluids, containing magnetically-soft abrasive particles dispersed in a liquid carrier, exhibit magnetically-induced thixotropic behavior in the presence of a magnetic field. The apparent viscosity of the fluid can be magnetically increased by many orders of magnitude, such that the consistency of the fluid changes from being nearly watery to being a very stiff paste. When such a paste is directed appropriately against a substrate surface to be shaped or polished, for example, an optical element, a very high level of finishing quality, accuracy, and control can be achieved.
U.S. Pat. Nos. 5,449,313 issued Sep. 12, 1995 and 5,577,948 issued Nov. 26, 1996, both to Kordonsky et al. disclose magnetorheological polishing devices and methods.
U.S. Pat. No. 5,525,249 issued Jun. 11, 1996 to Kordonsky et al. discloses magnetorheological fluids and methods of making thereof.
U.S. Pat. No. 5,616,066 issued Apr. 1, 1997 to Jacobs et al. discloses methods and apparatus for magnetorheological finishing of edges of optical elements.
U.S. Pat. No. 5,795,212 issued Aug. 18, 1998 to Jacobs et al., the disclosure of which is hereby incorporated by reference, discloses methods, fluids, and apparatus for deterministic magnetorheological finishing of substrates. This patent is referred to herein as "U.S. Pat. No. '212."
In a typical magnetorheological finishing system, such as is disclosed in the U.S. Pat. No. '212, a work surface comprises a vertically-oriented wheel having an axially-wide rim which is undercut symmetrically about a hub. Specially shaped magnetic pole pieces, which are symmetrical about a vertical plane containing the axis of rotation of the wheel, are extended toward opposite sides of the wheel under the undercut rim to provide a magnetic work zone on the surface of the wheel, preferably at about the top-dead-center position. The surface of the wheel may be flat, i.e., a cylindrical section, or it may be convex, i.e., a spherical equatorial section, or it may be concave. The convex shape can be particularly useful as it permits finishing of concave surfaces having a radius longer than the radius of the wheel.
Mounted above the work zone is a substrate receiver, such as a chuck, for extending a substrate to be finished into the work zone. The chuck is programmably manipulable in a plurality of modes of motion and is preferably controlled by a programmable controller or a computer.
Magnetorheological fluid having a predetermined concentration of particles which are magnetically soft is extruded in a non-magnetized state, typically from a shaping nozzle, as a ribbon onto the work surface of the wheel, which carries it into the work zone where it becomes magnetized to a pasty consistency. In the work zone, the pasty MRF does abrasive work on the substrate and becomes heated thereby. The heating and exposure of the MRF causes some evaporation of carrier fluid and a consequent concentrating of the MRF. Exiting the work zone, the concentrated fluid becomes non-magnetized again and is scraped from the wheel work surface for recirculation and reuse.
Fluid delivery to, and recovery from, the wheel is managed by a closed fluid delivery system. MRF is withdrawn from the scraper by a suction pump and sent to a tank where its temperature is measured and adjusted to aim. Recirculation from the tank to the nozzle, and hence through the work zone, at a specified flow rate is accomplished by setting the speed of rotation of a pressurizing pump, typically a peristaltic pump. The concentration of solids in the MRF as discharged onto the wheel is an important factor in controlling the rate of abrasive milling of a substrate in the work zone. Viscosity being a direct correlate of concentration, it is highly desirable to dynamically adjust the viscosity of the concentrated MRF being recirculated to an aim value during use. In the line between the pump and nozzle is a viscometer comprising a length of capillary tubing having upstream and downstream pressure sensors. At a constant fluid flow rate, the pressure drop through the capillary tubing, that is, the pressure difference between the two pressure sensors, is proportional to the viscosity of the fluid. An increase in pressure drop is inferred to mean an increase in viscosity and is used to cause replenishment of carrier fluid into the MRF in the tempering tank to reduce the apparent viscosity to aim.